In portrait shooting or in close-up shooting of plants etc., photographic expression where a main subject is risen up by intentional blurring of the foreground or the background thereof, has been generally preferred. When such photographic expression is used, shooting is performed in a state where depth of the field is very shallow and the main subject is focused. Here, the depth of the field is determined depending on optical elements such as the aperture or the focal-length of a lens.
On the other hand, the size of the exposure plane of an image sensor of a commonly used electronic camera is remarkably small as compared with that of the exposure plane (film) of a so-called silver salt camera. For this reason, if the same field angle as that of the silver salt camera is intended to be obtained, the focal-length of a shooting lens of a digital camera will be shorter than that of a shooting lens of the silver salt camera. Moreover, since a shooting lens with a large aperture diameter is large and expensive, in commonly used electronic cameras, a shooting lens with a small aperture diameter is used in many cases. Therefore, the depth of field of a commonly used electronic camera tends to be deeper than that of the silver salt camera, thus, in many cases, it is difficult for the common electronic camera to make photographic expression utilizing a blurring effect as mentioned above.
Therefore, technologies for generating a portrait photographic tone image by way of digital image processing have been disclosed in Patent Document 1 and Patent Document 2.
However, in the technology of Patent Document 1 mentioned above, blurs of subjects except for a main subject are emphasized depending on the distances between each of the subjects and the camera by measuring distances of a plurality of places for one shooting. Therefore, there has been a room for improvement in that a distance measuring units capable of measuring a plurality of places is required, thus resulting in complicated camera configuration.
On the other hand, in the technology of Patent Document 2 mentioned above, a contrast ratio is obtained from images having different focal-depths obtained via a plurality of optical shooting systems, and, based on the contrast ratio, a parameter of a low pass filter for adding an electrical blur is set. However, in order to obtain the images having different focal-depths, a plurality of optical shooting systems is required. For this reason, there has been a room for improvement in that camera configuration becomes complicated and large in size.
Moreover, in order to attain miniaturization of a camera, when the above operation is intended to be achieved by a camera having one optical shooting system, it is necessary that, while maintaining a focused state, shooting is performed twice by changing an aperture value, and each of the photographic images is subjected to processing. Therefore, there is a problem of processing time.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H 11-266 388    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-101